This invention relates to multi-purpose photographic film cassettes and more particularly, it concerns improvements in film cassettes of the type in which a film strip, connected at opposite ends to supply and take-up spools rotatably supported in the cassette, is exposed, processed and projected without removal of the film strip from the cassette.
In motion picture systems of the type now available commercially under the tradename "POLAVISION" from Polaroid Corporation of Cambridge, Massachusetts, a photographic film strip is permanently contained in a multi-purpose cassette which is inserted into a camera for exposure of the film strip and then transferred to a viewing device or "player" in which the film strip is processed and projected for viewing. Processing of the film strip without removal from the cassette is achieved by a cassette-contained supply of processing fluid and a processor capable of depositing a very thin layer of processing fluid on the inner emulsion side of the strip as it is removed from the take-up spool (on which it was wound during exposure) to the supply spool. The processing operation is followed directly by a mandatory projection cycle to assure complete drying of the fluid and/or layers of the film strip which are wet by the fluid.
The film strip currently used in the system is an additive color film structure including, in the order of light transmission during exposure, a transparent polyester carrier base, an addititive color screen, a processing fluid barrier layer, a positive image receiving layer or interface, a silver halide emulsion and an inner layer of antihalation dyes and image stablizer. When processed by coating the inner layer with a thin uniform layer of processing fluid or liquid, the chemicals contained in the processing fluid permeate the inner layer to the emulsion to develop exposed silver halide grains and render them essentially transparent. Unexposed silver halide grains migrate by diffusion to the positive image-receiving interface at which they are transformed into an opaque silver image in varying degrees of density. During and after development of the images, the antihalation dyes in the inner layer are bleached to become colorless. The negative image in the emulsion is sufficiently low in covering power relative to the positive image that the film strip may be viewed by projection of light in a direction proceeding through the inner layer, the processed emulsion layer and the remaining positive image carrying portions of the film strip and the color screen.
Though acceptable in the present commercial version of the "POLAVISION" system, the presence of the spent inner and emulsion layers on the film strip after processing and during projection can be characterized as a compromise between advantages to be gained by removal of these layers and difficulties heretofore experienced in achieving their removal while retaining the desirable characteristics of the present system, particularly those characteristics attributed to the retention at all times of the film strip in the same cassette in which it was initially packaged. A principal advantage to be gained by removing the spent inner and emulsion layers is greater transmissibility of the processed film during projection. In addition, because these layers are wet by the processing fluid immediately in advance of the film strip being rewound on the supply spool in adjacent convolutions, some provision, such as marginal side rail elevations on the film strip as in the present commercial version, must be made to separate the wet inner surface of the film strip and the adjacent face of a preceding convolution on the supply spool.
The problems resulting from retention of the spent inner and emulsion layers on the processed "POLAVISION" film strip have been recognized previously and dealt with by stripping these layers from the film after processing. See, for example, U.S. Pat. Nos. 3,455,633 and 3,711,192 issued to Edwin H. Land on July 15, 1969 and Jan. 16, 1973, respectively. While the structure of the film strip thus lends itself to stripping or removing spent inner and emulsion layers so that only positive image forming components are physically present on the film strip for projection, the difficulty in achieving the stripping operation without removal of the film strip from the cassette have been a deterrent to the actual practice of emulsion removal.